04 volatile keyword implementation principle

The principle volatile keyword

1, the semantic analysis of the volatile keyword

Role: let other threads can immediately perceive a thread of a multi-variable changes

• Ensure the visibility of
  
  changes to shared variables, other threads can immediately perceive
• Ensure orderly
  
  guarantee volatile modified the code in the same position when reordering

• The principle
  
  will be taken from the main memory directly For that volatile variables will compile time using the Lock command, other threads to use Lock command modified variables

  2, volatile usage scenarios

• Status flag (switching mode)
  
  a thread by changing the value of the shared variable, to control the other end of the real
• Double-checked locking (double-checked-locking) DCL
  
  lazy single embodiment

• Need to use sequential

3, the difference between volatile and synchronized

• Using a
  
  volatile variable modifications, synchronized statement blocks and modification methods
• Atomicity guarantees
  
  synchronized to ensure the atomicity, volatile can not be guaranteed atomicity
• Visibility assurance
  
  can be guaranteed, but the principle of different
  
  volatile variables plus lock, synchronized using monitorEnter and monitorExit
• Guarantee orderliness
  volatile prohibited reordering, the ordering guarantees
  
  sychronized by adding heavyweight lock will degenerate into a serial parallel, easily causing obstruction